As for the relative power of Star Destroyers, at the start of the time period they were still relatively strong (ISDII at least, ISDI was already becoming outclassed). By the end of it they were pretty weak compared to most new capital ships.
As the empire of the Hand is a shadow empire of the actual empire, wouldn't they also have access to Star Destroyers?
They were funnelled some material and personnel by the Empire and the Chiss Ascendancy, especially during their initial formation, considering that it was formed by a task force under Thrawn. However, they ended up being much bigger than the Empire, which, after Endor, was barely able to sustain itself, much less a second Empire. There's no evidence that the Empire of the Hand ever produced Imperial designs itself, either. In the few times when we get to see EotH forces canonically, it's with their own designs, being the Chiss Star Destroyer (which we've named the Ascendancy in the mod), and Nsiss Clawcraft. They did, however, maintain the use of stormtrooper armour, although their Stormtrooper corps were open to all species, as opposed to just humans. Their ship designs were said to maintain strong visual links to Imperial designs in many, which we've also done. As far as military cooperation between them, the Empire of the Hand was really only shown in a few conflicts; the Redoubt, where they assisted the New Republic and worked with the Chiss Asscendancy, the Vong War, where they were mainly fighting on their own in the Unknown Regions, but were only shown directly assisting the New Republic, and at Exodo II, where Jag Fel used it against other Imperials. The fact that everyone who really knew about/supported its existence or connection to the Empire died (Palpatine and Thrawn, although Isard sent Thrawn Soontier Fel to keep him from ousting her) within a few years seems to have practically served to sever the two Empires after Endor.
As far as astrography controlling which ships factions tended to build, that doesn't seem to have been the case at all; there's evidently nothing particularly special about any Imperial/New Rep/other shipyards that's required to build Star Destroyers or any other ships really, as long as you had the designs for them and the desire to build them, which seems to almost never have been the case. The New Republic captured a handful of Imperial ships, but overwhelmingly designed and built their own. This is especially good for the mod, because this is above all a game, and having every faction build the same ships would mean the faction you pick is ultimately meaningless.